Why it takes so long for Joshua trees to grow
Joshua trees strike a mesmerizing figure across the landscape. Their spiny tops and every-which-way branches make them look like something from a fantasy picture book.
These iconic plants require some time, however, to reach that otherworldly appearance. They rely on a specific chain of events to achieve pollination, and from there, they grow in bursts — some slow-going, some not — but only under the right circumstances.
It’s important, however, that they do grow. Joshua trees play a vital role in desert ecosystems, so a loss of a Joshua tree — like the ones damaged recently at Joshua Tree National Park — is a loss for that environment.
No other moth but yucca
No species would feel the loss of the Joshua tree more deeply than the yucca moth. This equally fantastical-looking insect — it sports tentacle fronds instead of the long tongue common to other moths and butterflies — relies on the Joshua tree for habitats in which to lay its eggs and for food when those eggs hatch. Lest you think that the Joshua tree doesn’t get anything out of this arrangement, rest assured that it does. In fact, without the yucca moth, the Joshua tree couldn’t survive.
The yucca moth and the Joshua tree are intertwined species. (Photo: Joshua Tree National Park/Flickr)
Joshua trees don’t produce nectar and thus rely on the life cycle of the yucca moth to achieve pollination. Females gather up pollen from the Joshua tree’s blooms, holding it a little ball with its mouth tentacles. The moth goes in search of another flower on a different Joshua tree that doesn’t already have eggs on it. Once it finds one, the moth lays its eggs near the flower’s ovary and then deposits the pollen ball on the stigma. The female produces only a small number of eggs. If there are too many eggs, the flower will not produce the fruit necessary for when the eggs hatch.
Larvae eat only some this fruit once they hatch and then, once they’re fully grown, drop to the ground, bury themselves and form cocoons. There they will remain until next spring when the whole cycle begins again. The remaining fruit will disperse — either by the wind or by small desert mammals — to grow more Joshua trees.
Without one another, the Joshua tree and the yucca moth wouldn’t survive. Scientists consider the relationship between the two organisms one of the classic examples of co-evolution, with Darwin once calling it the “most wonderful case of fertilization” known.
Slow and old
So not only does the Joshua tree require the presence of the yucca moth, but it also grows slowly, thanks to its desert environment. Those dispersed seeds require “well-timed” rains to begin growing, according to the U.S. National Parks Service. It’s also important to have a good freeze during the winter. Researchers think the freezing temperatures damage the growing portion of the branch and stimulate both flowering and branching. Some seeds don’t get the rains and thus never develop while others don’t receive the winter snap. Those trees end up look like tall, slightly bulbous stalks that never bloom or grow branches.
Under the right conditions, however, the Joshua tree will grow, albeit at an odd pace. The U.S. Forest Service describes Joshua trees as “slow-growing and long-lived,” both of which are accurate. During its time as a seedling, a Joshua tree may grow around 3 inches (7.6 centimeters) a year for 10 years, depending on conditions. After that, growth slows to a crawl, with plants averaging 1.5 inches a year.
The trees can reach 20 to 70 feet (5 to 20 meters) in height, meaning the trees can live for hundreds of years provided conditions are right and they can survive the harsh desert landscape. However, determining a Joshua tree’s age is tricky. The plants don’t have tree rings, and thus we can only estimate a plant’s age based on its height.
And the desert relies on these plants reaching maturity and lasting for a long time. Joshua tree branches provide nesting sites for the Scott’s oriole, while the spiny bases of the plant provide a built-in security system for wood rats that build nests at the base of the Joshua tree with rocks. The branches also provide shade for ground animals during the day, a handy way to beat the desert heat.
Threats from all side
The sun may set on Joshua trees if we’re not careful. (Photo: Kristina D.C. Hoeppner/Flickr)
Given their importance and their slow growth, the status of Joshua trees is always on the minds of conservationists and people who simply love the trees.
Climate change, for instance, threatens the trees’ environment. The desert soil is losing moisture that the trees and other organisms need to survive as temperatures increase and rainfall decreases. This means those seeds will struggle to reach maturity.
“A lot of times when people look at a place like Joshua Tree National Park where you see a lot of mature trees, they think it looks healthy,” Cameron Barrows, an ecologist at the Center for Conservation Biology at the University of California, Riverside, told Smithsonian in 2017. “But if you’re not seeing the juveniles, that means the species isn’t replacing itself.”
Joshua trees, it seems, are trying to migrate north, but this will take generations and thousands of miles to accomplish. Additionally, the trees will require the ever-important yucca moth to migrate with them. Scientists don’t know how the moths will respond to such a shift in climates.
Another potential harm to the Joshua tree’s survival? Us. During the 2018-2019 federal government shutdown, Joshua Tree National Park lacked the rangers necessary to keep the park protected and cleaned. When the park re-opened in late January, rangers and conservationists found new roads in the park created by unauthorized off-road expeditions and that a small number of Joshua trees had been destroyed in that process.
Destroying the plants not only hurts the environment but hurts the existence of the plant as a species. Protecting these wondrous plants is important not only for their beauty but also for their role in supporting life in the desert.
Why it takes so long for Joshua trees to grow
Joshua trees need a certain insect and a lot of time for pollination and growth, making them beautiful but fragile trees.